Trim fat from roast. In a small bowl combine cumin, chili powder, salt, and black pepper. Sprinkle evenly over roast; rub in with your fingers. If necessary, cut roast to fit into a 4- or 4-1/2-quart slow cooker. Place roast in slow cooker. Add onion, undrained tomatoes, green chile peppers, and, if desired, jalapeno chile peppers.

Cover and cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours. Transfer roast to a cutting board, reserving juices in slow cooker. Using 2 forks, gently separate the meat into thin shreds. Return meat to slow cooker; heat through. Stir in cilantro.

To serve, sprinkle cheese over bottoms of onion or kaiser rolls. Using a slotted spoon, place about 1/2 cup of the meat mixture on top of cheese on each bun. If desired, add a lettuce leaf to each. Replace roll tops. Makes 16 servings.

*NOTE: Because chile peppers contain volatile oils that can burn your skin and eyes, avoid direct contact with them as much as possible. When working with chile peppers, wear plastic or rubber gloves. If your bare hands do touch the peppers, wash your hands and nails well with soap and warm water.